Such a reversible printing unit has been known, e.g., from Swiss Patent Specification No. CH-PS 557,238. Two central distributing rollers, which can be connected via reversible transfer cylinders, are arranged in the inking mechanism of this printing unit. The first central distributing roller is inked via a number of inking rollers, which receive ink from a ductor roller. The second central distributing roller receives ink from a transfer cylinder, which comes into contact with both central distributing cylinders. The other transfer cylinder connects the second central distributing roller to another distributing roller, which transfers the ink to an applicator roller. A distributing roller, which in turn is in contact with another applicator roller, is arranged symmetrically to the second central distributing roller. The two transfer cylinders are mounted rotatable on angle levers pivoted around the second central distributing roller. The two transfer cylinders are continuously in contact with the second central distributing roller. In a first position, a transfer cylinder connects the first central distributing roller to the second central distributing roller, while the other transfer cylinder connects the second central distributing roller to another distributing roller. The symmetrical distributing roller and the corresponding applicator roller are excluded from the ink flow, and they assume the task of dampening, because one dampening mechanism is associated with each of the two symmetrical distributing rollers. When the transfer cylinders are pivoted together, the ink flows via the other distributing roller, while the first distributing roller assumes the dampening function.
The plate cylinder can always be pre-dampened with this device, i.e., the dampening agent is applied by the ink to the plate cylinder, even when the direction of rotation of the plate cylinder is reversed. This pre-dampening is desirable in some cases.
The configuration of the inking and dampening mechanism permits only this type of supply of the dampening agent for the plate cylinder, which is the so-called direct dampening.
Indirect dampening, i.e., application of the dampening agent to the plate cylinder and introduction of the dampening agent into the inking mechanism, is impossible with this arrangement.
When changing from one printing ink to another, the inking mechanism must be cleaned, which normally takes place automatically. The rollers of the dampening mechanism must be cleaned as well. To do so, a washing device must be brought into contact with e.g., one of the central distributing rollers. To permit cleaning of all rollers of the inking and dampening mechanism, cleaning have must be performed via the plate cylinder. However, this has the disadvantage that rapid and thorough cleaning of the rollers of the inking and dampening mechanism is not possible.